Proper control valve and relief valve selection is critical for profitable LNG terminal operations. Jean-Paul Boyer, Massimiliano Franco, Nagendra Maddula, and Eugenio Sudati, Emerson, explain the key design parameters for choosing the best components.
LNG facilities present punishing problems for valves. The combination of cryogenic temperatures, two-phase flow, very high pressures, and cyclic pressure and temperature swings push these components to their very limits. Yet, despite these conditions, the valves must perform reliably, or the entire operation suffers.
This article examines some of the more common valve applications across LNG storage and regasification facilities, and discusses the critical design features when selecting valves for this service.
The LNG process
LNG is a cleaner alternative compared to most other fossil fuels, and it has become a mainstay of the low-carbon energy market. Natural gas is purified, sub-cooled, and liquified at approximately -160°C, then shipped around the globe to locations near its point of use. There, the LNG is pumped into storage tanks, and then vaporised into gas as needed to be transferred to other sites via pipeline or supplied to local users (Figure 1).
The process depicted in Figure 1 shows a closed-loop LNG boil-off gas (BOG) system where BOG compressors are used to reliquefy gas vapours moved from a ship to LNG storage tanks. The tanks must be continuously kept under very low pressure, so BOG compressors continually pull vapours off the top of the tank. LNG recondensers remove heat and convert the vapours back to liquid, avoiding unnecessary flaring and waste. Ultimately, the LNG is vaporised, and then either pumped into a pipeline for overland transport, or directly fed to local users.
Figure 1. The unloading, storage, and regasification of LNG involves a variety of very difficult and challenging valve applications, each critical for efficient operation.
On/off valves in LNG service
On/off valves play a critical role across the entire LNG facility. Manual valves are used to isolate various pieces of equipment and must provide zero leakage, despite operating under very high pressures and cryogenic conditions. Automated valves provide diversion of LNG liquid and vapours, and they are often required for safety shutdown applications.
Any valve chosen for this application must first be designed to handle the process conditions, which are typically cryogenic. Cryogenic valves usually have extended bonnets to separate the valve from the actuator, and they often employ quarter-turn styles and special environmental packing designs to reduce emissions. The valves must not trap liquids, so C-ball or high-performance triple offset valves are commonly used because they provide zero leakage, even when pressurised from either side. This is usually accomplished using metal-to-metal torque seating, along with special materials of construction suitable for the wide range of temperatures the valve may encounter. Top-entry body styles allow automated valves to be maintained while still installed, dramatically reducing the time required for valve repair.
Safety emergency shutdown valves can be specified with a very fast stroke time needed to achieve the required performance. These safety integrity level (SIL)-rated solutions employ dedicated on-board components and high-performing positioner designs capable of controlling the speed at the end of the stoke, typically referred to as reseating.
LNG vapour temperature control
LNG is often injected into vapour streams to provide cooling on the suction of the compressors, as well as in the vapour line used to offload the LNG carrier. The LNG injection application is a difficult one and usually employs a specially designed cryogenic de-superheater valve.
This valve employs an extended bonnet to protect the actuator, along with carefully designed spray nozzles to provide complete atomisation across a wide range of liquid and vapour flows. The valve must also provide very tight shutoff when not in operation to avoid leaking liquid into the vapour line and creating two-phase flow conditions. The valve body should be designed to allow easy maintenance and replacement of the nozzles as they wear.
LNG compressor anti-surge valves
Centrifugal compressors are prone to a catastrophic condition called surge, which occurs when either the suction intake or discharge outflow of a compressor is blocked or restricted. When this happens, the compressor can suddenly be subjected to a destructive condition where the vapour slams forwards and backwards through the compressor several times a second. A few surge cycles will damage the thrust bearings, and if allowed to continue, surge will destroy the internals of the compressor.
Surge is avoided by installing an anti-surge control valve that shunts the compressor discharge back to the suction to keep flow moving through the machine.
The anti-surge valve must be sized large enough to pass a significant portion of the compressor flow. It must also be very fast acting, moving from 0% to 100% open, or any position in between, in under 2 secs., while maintaining precise control. This is accomplished using high-capacity and high-precision positioners, dedicated linear actuators, pneumatic boosters, and air volume tanks. The entire assembly must be carefully designed, and then undergo extensive testing prior to installation to ensure proper and reliable operation.
Anti-surge valves must provide very tight shutoff in normal operation, then instantly pass a tightly controlled flow, despite very high pressure drop. Some valves are specifically engineered with a trim dead band to allow partial stroke testing without passing significant vapour flow, increasing uptime as compared to performing full stroke testing, which must be done when the unit is offline.
LNG storage tank pressure relief
LNG storage tanks hold large amounts of LNG at cryogenic temperatures. If that temperature control is lost, or if the BOG compressors fail, the LNG will start vaporising and subject the tank to significant over-pressure in a very short time. Therefore, these tanks must be protected by one or more storage tank pressure relief valves (Figure 2). Very large capacity is a must for these valves, or each tank will require a large number of valves for adequate protection. High-quality, pilot-operated designs are necessary to provide high capacity and suitable tightness, and they can be specified with either pop or modulating performance to meet the application requirements. The pilot-operated design also allows the relief valve to handle backpressure from flare headers. Some designs offer the option of wireless monitoring to detect open or leaking relief valves, providing plant personnel with the data required to dramatically reduce product loss and environmental releases.
Figure 2. Tank relief venting systems, including components such as Emerson’s Anderson Greenwood 9300H pilot-operated safety relief valve, must maintain a bubble-tight seal to within a few percent of setpoint, then pass very high volumes of both LNG liquid and vapour when called into operation.
LNG pressure protection
In addition to the pressure relief vents on the storage tanks, pressure protection devices are used across an LNG facility to protect equipment from overpressure. Any time LNG is trapped between two valves, it can quickly begin to vaporise and generate extremely high pressures. Small thermal relief valves protect the piping and valve seals by venting this vapour. In other applications, pilot-operated relief valves provide over-pressure protection for all types of tanks, exchangers, and various process equipment.
Relief valves in LNG service must be carefully designed to handle the two-phase flow and cryogenic temperatures common in these applications. The valves must also seal very tightly, even when subjected to pressures near the setpoint, then consistently and reliably open when the relief setpoint is reached.
Modulating pilot designs provide the minimum venting required to relieve the overpressure condition, and they have the capacity to relieve very high flows when necessary. They are also insensitive to discharge backpressure. Like the tank relief valves, these pressure relief valves can be provided with wireless monitoring to detect leaking or venting valves. This can be critical when the valve vents into a flare header, as leaks and venting operations are not easily detected in these types of applications.
It is important to understand the key design features when tasked with selecting valves for LNG service. It is also critical to choose valve designs that have a proven track record of consistent and reliable performance for a given application. The LNG process subjects valves to very difficult and punishing conditions, and the price of failure can be very costly and damaging to equipment.
When considering options, it is wise to consult control valve and relief valve automation partners to evaluate the available designs and select the best option for a particular application. Careful selection can significantly extend service life, reduce downtime, dramatically reduce emissions, and improve plant profitability.
Read the article online at: https://www.lngindustry.com/special-reports/22112022/the-key-to-valve-selection-for-challenging-lng-applications/
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